An adhoc network is an autonomously configured type of network linked by wireless or wired communication. The adhoc network is configured by a plurality of nodes. Each node in the adhoc network transmits and receives packets using multihop communication. Multihop communication is a technology that allows nodes that are not present in the same communication zone to communicate with each other through another node that is present in the communication zone of the nodes.
Systems that use adhoc networks include a system that is able to conduct meter reading operations and the like through the adhoc network without workers traveling to the sites of the meters, by building nodes that enable wireless communication in household electrical power meters. This type of system is expected to enable secure communication (communication using encryption technology) from the standpoint of confidentiality and falsification prevention since private information such as the electrical power utilization amount of a household is managed in the adhoc network.
Conventional adhoc communication systems use a method that involves encrypting packets transmitted and received between the nodes in the adhoc network and adding a message authentication code (MAC) that is a type of encoding information to the packets. Such a method is disclosed in, for example, Japanese Laid-Open Patent Publication No. 2003-348072, Japanese Laid-Open Patent Publication No. 2010-98597, Japanese Laid-Open Patent Publication No. 2007-88799, and Japanese Laid-Open Patent Publication No. 2009-81854.
Regular packets sent and received in an adhoc network may be captured by anyone inside an adhoc network. As a result, attackers retransmit captured regular packets in an adhoc network in order to be able to easily execute an attack (retransmission attack) by congesting the network. Adhoc networks are desirable prepared against retransmission attacks by ensuring communication quality in the network since the abovementioned encryption of packets and the addition of message authentication codes in themselves are not able to reduce retransmission attacks.
Therefore, transmission time information of the packet is held in the header, and there is a technique that compares the time that the packet was received by the node with the transmission time in the header. If a large difference is found, this discrepancy is considered as the execution of a retransmission attack and the packet in question is discarded. When a retransmission attack is executed and malicious packets are concentrated around a specific node in this technology, a large amount of the processing capability of the node is taken up by the processing to discard the packets. As a result, a processing delay occurs in the particular node and there is a problem that a processing delay for the entire adhoc network may occur. International Publication Pamphlet No. WO 2011/121713 discloses a technique to address this problem by dispersing the load of the node throughout the entire adhoc network by selecting at random header verifications and message authentication code verifications when a node transmits received packets to another node.